22 Feb 2013

Efficiency

I’ve recently (re)taken up cycling in a fairly major way, and have
been surprised by how much I’ve enjoyed it. One of the things that’s
making it more fun this time around, as compared to previous dabblings
in years past, is the various ways that you can measure and quantify
your progress – not to mention your suffering – and compare it with
others, etc.

Now, 120 W is really not especially great from a competitive cycling
perspective; better riders routinely output 500-ish watts. But it
struck me as being pretty efficient: for all my effort, the ride
actually only required the same amount of power to propel me on my way
as would have been required by two household light bulbs.

So that got me thinking: just how efficient is cycling?

My 25.8 mi / 41.5 km roundtrip ride required 826 kJ, if we believe
Strava; that’s mechanical energy at the pedals. (I unfortunately
don’t have a power meter on my bike, so this is a bit of an estimate
on Strava’s part, taking into account my weight, my bike’s weight, my
speed, the elevation changes on the route, etc.)

That’s about the same as the energy released by 1.7 grams of combusted
gasoline, per Wolfram Alpha. If I ran on gasoline, I’d be able to
carry enough in my water bottle to ride across the U.S. more than 3
times (7,813 miles worth).

Of course, cars aren’t perfectly efficient in their use of gasoline,
and I’m not a perfectly efficient user of food calories. Strava
helpfully estimates the food-calorie expenditure of my ride at 921
Calories, which is 3.85 MJ, leading to a somewhat disappointing figure
of only 21.4% overall efficiency. (Disappointing only in the
engineering sense; from an exercise perspective I’d really rather it
be low.)

Though it’s about on par with a car, interestingly enough. The
Feds give anywhere between 14-26% as a typical ‘tank-to-tread’
efficiency figure for a passenger car, with most losses in the engine
itself.

So if I were able to drink gasoline and use it at least as efficiently
as a car, my water bottle would get me about a thousand miles. (1,094
mi or 1,760 km, using the low-end 14% efficiency figure for a car.)
Still pretty good, considering that my own car would only get about 5
miles on the same amount of fuel (24 fl oz at 25 MPG).

Of course, a car isn’t an especially fair comparison – it has a lot
of overhead both in terms of mass, rolling resistance (more, lower-pressure
tires), and air resistance (higher cross-sectional area). Some sort
of small motorbike would be a better comparison, and there I suspect
you’d start to see an even playing field.